1. Field of the Invention
The present invention relates to a coordinate input apparatus called a digitizer or a tablet which is a type of input apparatus for a computer, and more particularly relating to a sensor coil pattern of a sensor surface.
2. Description of Prior Art
Regarding the coordinate input apparatus, various types of position detecting methods are generally known. Among these methods, an electromagnetic wave transmitting and receiving method is characteristic in that a position indicator may be cordless. FIG. 1 is a schematically illustrated structure of the coordinate input apparatus for explaining a basic operation thereof. In this method, the coordinate input apparatus is essentially composed of a position detecting surface (sensor surface) having many sensor coils arranged thereon in parallel with each other in position detecting directions and a position indicator such as a stylus or a cursor having a coil or a resonance circuit incorporated therein.
Normally the coordinates are detected in the directions of X- and Y-axes and therefore a pair of sensor surfaces are provided in the way as are crossed with each other at right angles in the directions of X- and Y-axes. In detecting the positions, the electromagnetic interaction is utilized between the sensor coils and the coil or resonance circuit provided in the position indicator.
More particularly with high frequency signals applied to a sensor coil, electromagnetic waves are produced at the sensor coil and then the resonance circuit in the position indicator will resonate due to the electromagnetic waves. In the next place when the delivery of the electromagnetic waves is stopped, the responsive electromagnetic waves are produced at the position indicator and then the sensor coil receives the responsive electromagnetic waves. Such signal received by the sensor coil is transmitted to a signal processing section through a signal receiving circuit.
Such operation is repeated while the many sensor coils on the sensor surface are switched over. With the distribution of the received signals thus obtained, the coordinates are determined while calculations are performed for complementing between the coordinates on the basis of the signals of a coil which is most strongly responsive and the signals of the coils in the neighborhood of the most strongly responsive coil. The detail of this electromagnetic transmitting and receiving method is disclosed in Japanese Patent Application Publication No. H2-53805.
Another position detecting method includes a more simple one in which the electromagnetic waves are delivered from the sensor surface to the position indicator or vice versa.
According to such position detecting method, so many sensor coils have to be scanned, that is, have to be progressively switched over to detect the positions. It is therefore time consuming. It takes longer time especially if the apparatus is comparatively bigger. At least it becomes necessary to scan all of the sensor coils in order to firstly seek the approximate positions of the position indicator.
Therefore if the scanning time is longer, a high speed position detecting operation is deteriorated. In consideration of this matter, a sensor coil pattern has been proposed in which a predetermined number of sensor coils are selected from the so many sensor coils and the selected sensor coils are connected with each other so as to simultaneously deliver and receive signals thereby to reduce the switching times thereof and thus to reduce the scanning time of all the sensor coils. One example of this method is disclosed in Japanese Patent Application Publication Laid-open No. H3-147012. FIG. 2 schematically shows a pattern which is disclosed in this patent application in which a predetermined number of sensor coils are connected with each other on the sensor surface 100. For the sake of simplification, the sensor coils are shown only in the direction of X-axis. The things are the same with respect to the Y-axis.
As shown below at the right end in FIG. 2, each sensor coil has a shape of two windings to increase sensitiveness and to obtain bigger signals. However in FIG. 2, for the sake of simplification, all the sensor coils are shown as of a single winding. According to the sensor coil pattern in FIG. 2, so many sensor coils are divided into a plurality of groups (though only three groups I, II, III are shown). Each group is composed of eight sensor coils. Further one sensor coil is selected from each group. The selected sensor coils are connected with each other. For example, the sensor coil 12 of group I, the sensor coil 21 of group II and the sensor coil 30 of group III are connected with each other.
One time signal transmitting and receiving operation is such that an alternate signal is simultaneously applied to the so connected sensor coils to produce electromagnetic waves thereat. Then if the position indicator is positioned in the neighborhood of any of the sensor coils, the electromagnetic interactions are performed between the position indicator and the sensor coils. Then the position indicator receives signals simultaneously from the so many connected sensor coils. For example, in reference to FIG. 2, three sensor coils 11, 22, and 33 connected with each other are simultaneously selected. A signal delivery and reception switch is provided to switch over the sensor coils from a signal transmitting condition to a signal receiving condition and vice versa. In the signal transmitting condition, the sensor coils are connected to a signal producing section. In the signal receiving condition, the sensor coils are connected to a signal detecting section.
The one time signal transmitting and receiving operation as above mentioned is repeated by progressively Switching over the terminals of a sensor coil switch 110. Thus in case that the sensor coils are divided into groups, the sensor coil switching times are remarkably reduced when compared with the case in which the sensor coils are not divided into groups. According to the example of FIG. 2, the sensor coil switching times are reduced to one third.
In this method, with the only one time of signal transmitting and receiving operation, it is actually difficult to discriminate which of the mutually connected plural sensor coils are producing which of the signals to be obtained. However if the sensor coil switch is switched over in the order of the terminal numbers 0 to 7, the sensor coils of group I will be selected in the order of 10, 11, 12, 13, 14, 15, 16, 17, the sensor coils of group II will be selected in the order of 20, 22, 21, 24, 23, 26, 25, 27, and the sensor coils of group III will be selected in the order of 31, 33, 30, 35, 32, 37, 34, 36. Thus the sensor coils are connected with each other in the condition that the sensor coils in each group are made distinctly different from those of the other groups with respect to the arrangement and selection order thereof. Therefore with analysis of the received signal pattern formed with the received signals obtained from the mutually connected plural sensor coils and arranged in the receiving order, it becomes possible to discriminate in which of the sensor coil groups the position indicator exists. The detail of this method is disclosed in Japanese Patent Application Publication Laid-open No. H3-147012.
The above description concerns one example of sensor coil pattern which is composed of sensor coils selected from so many ones and connected with each other. Such pattern structure is practically very useful and therefore other sensor coil patterns have been proposed. The present invention relates to a specific sensor coil pattern which is composed of sensor coils selected from so many sensor coils and connected with each other.
The following description concerns the problems to be solved of the conventional coordinate input apparatus and the sensor coil pattern thereof especially in connection with the apparatus installing environments.
FIG. 3A schematically shows an example of a structure of liquid crystal display (LCD) having incorporated therein a coordinate input apparatus which is essentially composed of a sensor surface 100 and a stylus 130. In this structure, an LCD unit is placed at the upper end. The LCD unit is a laminated body composed of a liquid matrix, a transparent electrode, a back light, etc. For the back light, an FL is used and an electric discharging tube is provided at one edge of the unit. A side light may be used in place of the back light. A sensor plate 100, which is the sensor surface, is placed at the rear side of the LCD unit. A flat cable is extended out of one end of the sensor plate so as to deliver and receive signals thereat. Further a shield plate is placed at the rear side of the sensor plate.
A voltage of about 1000 volts is required to drive the electric discharging tube of the LCD back light, and therefore an inverter 140 is normally provided near the electric discharging tube in order to produce such voltage. The inverter 140 is normally a circuit for producing the alternate voltage of 5-12 volts up to about 1000 volts. Such inverter circuit is liable to produce a noise. Therefore the other circuits placed near the inverter circuit will have an adverse influence inviting the noise to the input lines thereof due to the electromagnetic connections even if the lines are not directly connected to the inverter circuit.
FIG. 3B is provided to show the adverse influence of the noise produced from the inverter 140 to the sensor coils on the sensor plate 100. In case the inverter 140 is placed as shown in this figure, the sensor coil 10 at the end adjacent to the inverter is most influenced by the inverter. FIGS. 5A to 5C show the shapes of received signals in various conditions. FIG. 5A shows a most preferred shape of received signals. FIG. 5B shows a shape of waves of received signals appearing on the sensor coil positioned adjacent to the noise source of inverter. Such signal waves have, as shown, barbel-like jitters superposed thereon.
In reference to FIG. 3B again, the problem is that the sensor coil 10 positioned at the end is electrically connected to another sensor coil 20 at the position spaced from the end. In case the noise as shown in FIG. 5B is produced at the sensor coil 10, the same noise will be transmitted to the sensor coil 20 which is electrically connected to the sensor coil 10. As the result, the received signals obtained from the sensor coil 20 will be observed as the waves as shown in FIG. 5B. It is needless to say that if such waves appear at the sensor coil 20, the exact position detecting operation will be adversely influenced. In FIG. 3B, there is no inverter adjacent to the sensor coil 3 positioned at the opposite end. In this case, there is no problem if the sensor coil 37 is connected to the sensor coil 27.
FIG. 4A shows a sensor plate installing environment having another problem involved. This is the case in which the LCD unit is surrounded by an electrically closed metal frame M. An influence of the metal frame M applied to the sensor coils will be explained by means of FIG. 4B. When the stylus 130 is on the sensor coil 10, an electric current IC will flow through the sensor coil 10 due to the magnetic flux produced at the stylus 130. The metal frame M, which constitutes a coil too, will produce an induced current IM which negates magnetic flux of the coil current IC. In short, the induced current IM will weaken the magnetic flux produced at the stylus 130. As the result, the intensity of the received signals obtained from the sensor coil 10 will become weaker than that of the received signals which may otherwise be normal. FIG. 5C shows a wave shape in comparison with a most preferred wave shape of FIG. 5A. This phenomenon is most remarkable when the sensor coil 10 is positioned adjacent to the metal frame M.
Here it is also a problem that the sensor coil 10 is electrically connected to the sensor coil 20. The above mentioned phenomenon will also appear when the stylus 130 is on the sensor coil 20. The reason is that the received signals produced at the sensor coil 20, that is, the coil current will flow through the sensor coil 10. Then the coil current will cause the metal frame M to produce an induced current thereat which will negate the coil current of the sensor coil 20 when the current flows through the sensor coil 10. In FIG. 4B, the sensor coil 37 positioned at the opposite end of the arrangement will be also subjected to the same influence. As the result, the sensor coil 26 electrically connected to the sensor 37 will be also subjected to the influence and the intensity of the received will become weaker.
The above mentioned two problematical installing environments concerning the coordinate input apparatus may often concurrently exist. In this case, the received signal property will be further deteriorated due to the doubled adverse influences. Namely a noise will be produced in addition to the reduction of signal intensity and accordingly the S/N rate will be remarkably worsened.